Control circuits



June 15, 1937. D. M. TERRY CONTROL CIRCUITS 2 Sheets-Sheefl Filed April'23, 1936 nwe/vron D. M. TERR V 7/1 /42M% ATTORNEY June 15-, 1937. b.TERRY 2,084,115.

- CONTROL CIRCUITS Filed April 23, 1936 2 Sheets-Sheet 2 FIG? I0 os r ByE A 7' TORNE V Patented June 15, 1&3?

UNITED STATES casein CONTROL CIRCUITS Donald M. Terry, Brooklyn, N; 3L,assignor to Bell Telephone Laboratories, Incorporated, New York, N. 53.,a corporation of New York Application April 23,

14 Claims.

This invention relates to gain control circuits for carrier signalsystems and particularly to protective circuits for gain controlcircuits.

One object of the invention is to provide a gain control circuit for asignal transmission line with protective circuits that shall prevent op-.eration of the gain control circuit in case the voltage of the signalson the line varies beyond predetermined limits.

Another object of the invention is to provide gain control circuits fora signal transmission line with protective circuits for preventingoperation of the gain control circuits in case the signals on thetransmission line vary beyond 15 predetermined limits and for operatingan alarm in case the signals on the transmission line vary beyondpredetermined limits for an extended period of time.

A further object of the invention is to provide gain control circuitsfor a carrier current transmission system with protective circuitscontrolled by a pilot current for preventing operation of the gaincontrol circuits in case the pilot current on the transmission systemvaries beyond predetermined limits and for operating an alarm in casethe pilot current on the transmission system varies beyond predeterminedlimits for an extended period of time.

In a carrier current signal system transmitting signals in one directionby means of carrier current within one frequency band and fortransmitting signals in an opposite direction by carrier currentslocated within another frequency band, the'energy level of the signalwaves ineach .115. frequency band is controlled by a pilot currenthaving a frequency located within the frequency band. The tran missionline particularly if an open wire transmission line is subjected tovarious conditions which cause variations in the 5-0 line attenuation.In wet weather the attenuation of an open wire line is greatlyincreased. In order to maintain the quality of the transmission it isnecessary to vary the gain of the signals on the line according to thevariation in 4: attenuation. A transmission line may under troubleconditions be subjected to abnormally large changes in attenuation forwhich no ordinary gain correction would be in order and it may also besubjected to so-called hits which so are momentary changes inattenuation.

According to the present invention a carrier telephone transmissionsystem having gain control circuits of the type disclosed in theapplication of J. W. Beyer and D. M. Terry, Serial No.

5.3 76,020, filed April 23, 1936, is provided with pro 1936, Serial No.76,021

tective circuits to prevent operation of the gain control circuits incase the strength of the signalsvaries beyond predetermined limits. Ifthe strength of the signals varies beyond predetermined limits for anextended period of time an alarm is operated.

In one form of the invention an amplifier in a signal'transmission linehas the input circuit thereof connected to attenuation networks in theline by means of a variable condenser. The variable condenser isoperated to raise or lower the gain by means of two synchronous motorswhich are selectively operated by a very sensitive relay. The sensitivemeter relay is controlled by a control circuit connected to thetransmission line at a point beyond the amplifier therein. The feedbackcircuit comprises means for selecting a pilot current and means foreffecting full voltage rectification of the pilot current.

The sensitive relay is provided with the two small permanent magnets forholding the contact arm of the relay in either of its operated positionsafter operation. Releasing means controlled by' the synchronous motorsis provided for returning the contact arm of the sensitive meter relayto initial or neutral position.

The feedback circuit which controls the sensitive relay governing thegain on the transmis sion line alsocontrols a relay of limitedsensitivity. The second relay will only be operated in case thevariationin the strength of the sig-' nals and the pilot current on thetransmission line is abnormal or beyond that which can be correctedunder control of the first relay. Upon operation of the second relay incase of a large change in strength of the pilot current on thetransmission line, means is provided for breaking the-circuit of the twosynchronous motors so that no control of the gain amplifier can beeffected by the highly sensitive meter relay. The relay of limitedsensitivity also controls means for operating an alarm in casethevariaticn in strength of the pilot current continues for an extendedperiod of time.

In the accompanying drawings,

Fig. 1 is a diagrammatic view of control circuits constructed inaccordance with the invention;

Fig. 2 is a diagrammatic view of a modification of the control circuitsshown in Fig. 1; and

Figs. 3 vand 4 are diagrammatic views of the sensitive relays shown inFigs. 1 and 2 of the drawings.

Referring to Fig. 1 of the drawings a transmission line comprising inputconductors I and 2 and output conductors 3 and 4 is provided with a gaincontrol amplifier 5 which is connected to a second amplifier '6 by meansof a transformer I. 'The transmission line shown in Fig. 1 of thedrawings may represent one channel of a repeater station as shown in theapplication of R. W. Chesnut, Serial No. 5,696, filed February 9, 1935now, Patent No. 2,049,195, issued July 28, 1936. Two attenuationnetworks 8 and 9 which may be of the type disclosed in the patent to R.W. Chesnut et a1. 1,880,800, dated October 4, 1932 are provided in thetransmission line for controlling the operation of the gain controlamplifier 5 in accordance with the operation of an adjustable condenserIll. The condenser I0 comprises a rotatable arm I I mounted on a shaftI2. The shaft I2 carries a plate I3. The condenser plate I3 is movableadjacent to condenser plates I4, I5 and I6. The condenser plate I4 isconnected to the line conductor I on one side of the network 6. Thecondenser plate I5 is connected to the conductor between the networks 8and 9. The condenser plate I6 is connected to the line conductor Ibeyond the network 9. A condenser arrangement of this type is disclosedand claimed in the application of C. R. Eckberg, Serial No. 5,717, filedFebruary 9, 1935.

The variable condenser I 0 is operated by a synchronous motor I! toraise the gain of the amplifier 5 and is operated by a synchronous motorI8 to lower the gain of the amplifier 5. The synchronous motor I!comprises a stator winding I9 and a rotor 20. The synchronous motor I8comprises the stator winding 2| and rotor 22. The rotors 26 and 22 ofthe motors I1 and I8 are mounted on a shaft 23 carrying a pinion 24. Thepinion 24 meshes with a gear wheel 25 mounted on a shaft 26. The shaft26 not only carries the gear wheel 25, but a cam member 21 and a pinion28. The pinion 28 meshes with a gear wheel 29 on the shaft I2 carryingthe contact arm II of the variable condenser I0.

The two synchronous motors I! and I8 which rotate in opposite directionsare selectively operated by two relays 3|] and 3| under control of asensitive relay 32. The sensitive relay 32, which is shown in detail inFigs. 3 and 4, comprises an operating coil 33, a contact arm 34 which isadapted to be moved into engagement with contact members 35 and 36 and arestoring coil 31. The contact members 35 and 36 are in the form ofsmall permanent magnets which engage and hold a small soft iron armature38 on the contact arm 34 whenever the contact arm is moved to either ofits operated positions. In order to restore the contact arm 34 toinitial or neutral position the restoring coil 3! is energized toattract a core armature 39. The core armature 39 efiects operation oftwo bell crank levers 49 and 4|. One or the other of the bell cranklevers 4B and 4t engages the contact arm 34 and moves it to neutralposition. An adjustable stop r 42 is provided for limiting the movementof thecore armature 39.

The relay 30 is provided with switch members 43 and 44 and relay 3| isprovided with switch members 45 and 46. The sensitive relay 32 isoperated by means of a feedback circuit connected to the conductors 3and 4 beyond the amplifier 6. The feedback circuit comprises twotransformers 4! and 48 which are tuned by means of condensers 49 and 50to select the pilot or control current on the transmission line and afull Voltage rectifier bridge 5|. The rectifier bridge 5| comprises therectifiers 52 and 53 forming two arms of the bridge circuit andcondensers 54 and 55 forming the other two arms of the bridge circuit.The output vertices of the bridge 5| are connected to the operating coil33 of the sensitive relay 32 and the input vertices of the bridge areconnected by the transformers 41 and 48 to the transmission line. Thetwo rectifiers 52 and 53, which are preferably of the copper-oxide type,are oppositely connected so as to efiect rectification of each half ofthe pilot current voltage. The sensitive relay 32 may be adjusted toeffect operation of the contact arm 34 by a half a decibel change in thestrength of the pilot current.

The feedback circuit also controls the operation of a relay 56 of lowersensitivity. The relay 56 which comprises an energizing coil 51, arestoring coil 58 and a contact arm 56 adapted to engage contact members60 and 6| is similar in construction to the sensitive relay 32. However,the sensitivity of the relay 56 is much lower when compared with therelay 32. If the relay 32 is set to be operated by a plus or minus onehalf a decibel change in the pilot current on the line, the relay 56will be set to operate, for example, at minus five decibels or plusthree decibel change of the pilot current.

The relay 56 of lower sensitivity directly controls the operation of arelay 62. The relay 62 by means of a switch arm 63 prevents energizationof either synchronous motors I! and I8 from a source of alternatingcurrent 640. under control of the sensitive relay 32. The relay 56 alsocontrols two heating cells 64 and 65 which, in turn, control the heatingof two silver sulphide elements 66 and 6?. The silver sulphide element66 is in the energizing circuit of a relay 68 and the silver sulphideelement 61 is in the energizing circuit of a relay 69. The relay 68 isprovided with switch arms 69a and Hi and the relay 69 is provided with aswitch arm II. The relay 69 when operated controls an alarm 12. Thealarm I2 is released by means of a relay I3 having a contact arm I4under control of a key I5.

If the strength of the pilot current on the conductors 3 and 4 islowered a half decibel, the strength of the rectified current suppliedto the winding 33 of the sensitive relay 32 is reduced to effectengagement between the contact member 35 and the contact arm 34. Acircuit is then completed from a battery I6 for operating the relay 39.The switch arm 43 when operated by the relay 3!! completes a circuitfrom the source of alternating current 64a for energizing the statorwinding I9 of the synchronous motor IT. The circuit completed by theswitch arm 43 may be traced from one terminal of the secondary windingof a transformer II through the switch arm 63 of a relay 62, switch arm43 of relay 30 and winding I9 to the other terminal of the secondarywinding. The synchronous motor I1 is operated to effect movement of thearm I I carrying the condenser plate I3 towards the left as viewed inFig. 1 of the drawings. This movement of the condenser plate I3 servesto raise the gain of the amplifier 5. Immediately thereafter the cammember 21 is operated to place a ground on contact members 36 and I9.contact member 13 by operation of the cam member 21 serves to complete aholding circuit for the relay 30 from the battery I6 through the switcharm 44. Grounding of the contact member I9 serves to complete a circuitfor energizing the Grounding of the restoring coil 31 of the relay- 3-2from a battery'8'0. The synchronous motor II will continue in operationunder control of the relay 38 until the cam member 2'! has completed onerotation to open: the holding circuit for the relay 30.

If the strength of the pilot current on the transmission line has notbeen raised to normal by the above operation the sensitive meter relay32 is operated again and the above cycle of operations is repeated. Ifthe strength of the pilot current goes more than a half decibel abovenormal value the sensitive relay 32 is operated by the feedback circuitto effect engagement between the contact arm 34 and the contact member36. This operation of the sensitive relay 32 completes a circuit frombattery 8| for operating the relay 3i The switch arm 48 of the relay 3icompletes a circuit from the alternating current source 54a forenergizing the winding 2I of the synchronous motor I8. The circuit forenergizing the winding 2i of the motor includes the switch arm '63 ofthe relay 82. The synchronous motor I8 when operated efiects movement ofthe condenser arm II in a direction towards the right as viewed in Fig.1 of the drawings to lower the gain effect by the amplifier 5.Immediately thereafter the cam member 21 places a ground on the contactmembers 18 and I9. Placing a ground upon the contact members 18completes a holding circuit for the relay 3! from the battery BI throughthe switch arm 45. The placing of ground upon the contact member 79completes a circuit from the battery 88 for operating the restoring coil31 of the relay 32. The restoring coil 37 returns the contact arm toneutral position in the manner above described.

In case of a minus decibel change from normal in the strength of thepilot current on a transmission line, the sensitive relay 32 will notonly be operated, but the relay 56 of limited sensitivity will beoperated. Upon operation of the relay 55 the contact arm 59 engagescontact member 68 for completing a circuit from a battery 82 foroperating the relay 82. The switch arm 63 operated by the relay 62prevents any operation of the synchronous motors I1 and [8 by thesensitive relay 32. The engagement of the switch arm 58 with the contactmember 68 also completes a circuit from a battery 83 for energizing theheating coil 54 which, in turn, controls the silver sulphide element 66.The silver sulphide element 88 when heated for a short period of time,for example, five seconds, completes an operating circuit from thesource of alternating current 88a for operating the relay G8. The relay38 operates the switch arm 89a for energizing the restoring coil 58 ofthe relay 58 from battery 84. The restoring coil 58 operates in themanner above described to return the contact arm 59 to initial orneutral position.

The contact arm 59 when in engagement with the contact member 58 alsocompletes a circuit for energizing the heating coil. 65 from a battery85. The heating coil 65 controls the silver sulphide element 6'! in thecircuit of the relay 58; The silver sulphide element '6'! does notcomplete the energizing circuit for the relay 89 until the element hasbeen heated for a longer period of time than the element 68, forexample, 25 seconds. The switch arm It also completes a circuit from thebattery 85 for energizing the heating coil 8-5 when the relay 88 isoperated. Thus, the heating coil 65 is energized either upon operationof the relay 55 or upon operation of the relay 68. If the disturbance onthe transmission line called hit and does not continue for an extendedperiod of time, then the relay 62 is released upon release of the relay56 and control of the gain on the transmission line is returned to thehighly sensitive relay 32. However, if the large change in strength ofthe pilot current in the transmission line continues for an extendedperiod of time, then relay 55 is repeatedly operated to finally heat thesilver sulphide element 81 suiiiciently to effect operation of the relay89. Upon operation of the relay 69 a circuit is completed from a battery88 for operating the alarm 72. The circuit for operating the alarm I2extends from grounded battery S'S-through the alarm 12, contact arm 14of the relay 13' and contact arm II of the relay 69 to the battery 86.The alarm I2 is stopped when the key 15 is closed to operate the relayT3. The relay 13 upon operation breaks the alarm circuit by means ofcontact arm M.

If the strength of the pilot current on the transmission line is raisedat least three decibels above normal, the relay 56 of lower sensitivityis operated and the contact arm 59 engages the contact member 85. Theoperation of the relay 56 and the protective circuits controlled therebyis the same as that given heretofore when the contact arm 59 engaged thecontact member 89 upon reduced level of the pilot current of thetransmission line. In the above operation it will be noted that when alarge change in the strength of the pilot line takes place in thetransmission line, the protective circuits prevent any operation of thegain control circuits and that if this change in the pilot currentcontinues for an extended period of time an alarm is sounded.

Referring to Fig. 2 of the drawings, a modification of the circuitsshown in Fig. 1 is shown wherein the protective circuits have beenchanged somewhat. The gain control circuits shown in Fig. 2 of thedrawings are exactly the same as the gain control circuits governed bythe sensitive relay 32 shown in Fig. l of the drawings. Accordingly,similar parts have been indicated by like reference characters.Moreover, a detailed description of the gain control circuits shown inFig. 2 will not be repeated.

The relay 55 of limited sensitivity shown in Fig. 2 of the drawingscontrols the operation of a synchronous motor 87 which is similar inconstruction to the synchronous motors II and I8. The synchronous motor81 comprises a stator Winding 88 and a rotor 89; The rotor 89 is mountedon a shaft 98 which carries a pinion 9!. The pinion 8i meshes with agear wheel 32 on a shaft 83. The shaft 93 also carries a pinion 84 whichmeshes with a gear wheel 95 on a shaft 86. The shaft 96 also carries cammembers 97, 98 and 99. The cam member 9'! controls two contact membersI88 and If for closing a holding circuit for the synchronous motor 81.The cam member 98 approximately 20 seconds after the start of theoperation of synchronous motor 81 places a ground upon contact membersI02 and I83. The cam member 99 closes contact members IM' and I85 about5 seconds after the grounding of the contact members I82 and I83 by thecam member 98.

In case a large variation in the strength of pilot current takes placeon the transmission line, the relay 55 of limited sensitivity will beoperated to cause the contact arm 58' to engage either the contactmember 69- or the contact member 8t. Upon engagement of the conwhicheffected operation'of the relay 56 is a sotact arm with one of thecontact members 69 and iii a circuit is completed from battery 86 foroperating a relay I01. A switch arm L93 operated by the relay l!prevents any operation of the synchronous motors I! and [8 which controlthe variation in gain effected by the amplifier 5. Switch arm I09operated by the re lay it? completes a circuit from the alternatingcurrent source of power 64 for energizing the Winding 88 of thesynchronous motor 81.' Switch arms H9 and HI operated by the relay I01form no useful function at this time. The synchronous motor 81 whenoperated effects rotation of the shaft 96 and the cam members 91, 93 and99. The cam member 91 when operated closes the contact members I60 andllil to complete a holding circuit for energizing the winding 88 of themotor 81 irrespective of the position of the relay WI. The holdingcircuit for the winding 88 of the motor 81 is held closed for onecomplete rotation of the shaft 98. Approximately 20 seconds after thestarting of the synchronous motor 81 the cam member 98 places a groundon the contact member 803 for completing a circuit from the battery 88to energize the restoring coil 31 of the relay 3i! and for completing acircuit from the battery 34 through the restoring coil 58 of the relay59. A ground is placed on the contact member IE2 at this time to insureoperation of the relay Ill! from the battery N36. The cam member 93approximately seconds after the grounding of the contact members i332and H13 closes the contact members i134 and N35 for completing a circuitto op erate a relay H2 from battery H3 if the relay it! is in operativeposition at this time. The circuit for operating the relay H2 may betraced from ground through switch arm ill of the relay l'a', switch armH4 of relay (3, contact members 165 and HM, relay H2 and battery IE3. Aswitch arm I I5 operated by the relay I I2 completes a holding circuitfrom the battery 85 for operating the alarm 12. A holding circuit forthe relay H2 is completed by a switch arm H6. The holding circuit forthe relay H2 may be traced from grounded battery H3 through relay !!2,switch arm H6, switch arm H4, switch arm iii and ground return to thebattery H3. The relay H2 may be released and the alarm stopped byclosing the key 15 which energizes the relay '53 for operating theswitch arm H4 to break the energizing circuit of a relay H2.

In the protective circuits above described no operation of thesynchronous motors l1 and it under control of the sensitive relay 32 cantake place when the energy level of the pilot current on thetransmission line varies sufiiciently to effect operation of the relay56. The relay 58 or the cam member 98 insures operation of the relay it?to prevent any operation of the relays and iii. If the large energychange in the pilot current continues for an extended length of time,the cam member 99 insures operation of the alarm '52.

Modifications in the circuits and in the arrangement and location ofparts may be made within the spirit and scope of the invention and suchmodifications are intended to be covered by the appended claims.

What is claimed is:

1. In combination, a signal transmission line, an amplifier on saidline, two relays of different sensitivity, means for operating saidrelays according to the strength of a pilot current on the line, meanscontrolled by the relay of higher sensitivity for controlling the gainof said amplifier to maintain the volume of the signals constant, andmeans controlled by the relay of lower sensitivity for preventingcontrol of the amplifier by the relay of higher sensitivity when thestrength of the pilot current varies beyond predetermined limits.

2. In combination, a signal transmission line, an amplifier spacedischarge device on said line, two relays of different sensitivity,means for operating said relays according to the energy level of a pilotcurrent on the line beyond said amplifier, means controlled by the relayof higher sensitivity for governing the gain of said amplifier tomaintain the volume of the signals constant, and means controlled by therelay of lower sensitivity for preventing control of the amplifier bythe relay of higher sensitivity when the energy level of the pilotcurrent varies beyond predetermined limits and for operating an alarm incase of large pilot current variations for an extended period of time.

3. In combination, a signal transmission line, an amplifier on saidline, a, condenser potentiometer for controlling the gain of saidamplifier, two relays of different sensitivity, means for operating saidrelays according to the energy level of a pilot current on the linebeyond said amplifier, means controlled by the relay of highersensitivity for operating said potentiometer to control said amplifier,and means controlled by the relay of lower sensitivity for preventingoperation of said potentiometer when the energy level of the pilotcurrent varies beyond predetermined limits.

4. In combination, a signal transmission line, an amplifier on saidline, a condenser potentiometer for controlling the gain of saidamplifier, a motor for operating said potentiometer, two relays ofdifferent sensitivity, means for operating said relays according to theenergy level of a pilot current on the line beyond said amplifier, meanscontrolled by the relay of higher sensitivity for operating said motorto control said potentiometer, and means controlled by the relay oflower sensitivity for preventing operation of said motor when the energylevel of the pilot current varies beyond predetermined limits.

5. In combination, a signal transmission line, an amplifier on saidline, means for controlling the gain of said amplifier according to theenergy level of a pilot current on said line, a relay of limitedsensitivity, means for operating said relay in case of large variationsin energy level of the pilot current and means controlled by said relayfor preventing control of said amplifier according to the pilot currentlevel changes and for operating an alarm in case of large variations inenergy level of the pilot current for extended periods of time.

6. In combination, a signal transmission line, control means governedaccording to the energy level of a pilot current on the line formaintaining the volume or" the signals on the line substantiallyconstant, a relay of limited sensitivity, means for operating said relayin case of large variations in energy level of the pilot current, andmeans controlled by said relay in case of large variations in energy ofsaid pilot current for preventing operation of said control means andfor operating an alarm when the variation in energy of the pilot currentcontinues for prolonged periods.

7. In combination, a signal transmission line, an amplifier on saidline, means for controlling the gain of said amplifier according to theenergy level of a pilot current on the line, a relay of limitedsensitivity, and means operated by said relay in case of largevariations in energy level of the pilot current for preventing controlof said amplifier according to the pilot level changes.

8. In combination, a transmission line having a condenser potentiometerfor controlling the gain of an amplifier, a motor for operating saidpotentiometer, two relays of difierent sensitivity, a control circuitconnecting said relays to said line beyond the amplifier device andcomprising a filter for selecting a pilot current and full voltagerectifier means for supplying rectified current to the relays, meanscontrolled by the relay of higher sensitivity for operating said motorto control the energy level of the signal on the line, and meanscontrolled by the relay of lower sensitivity for preventing operating ofsaid motor and for operating an alarm in case the voltage of the pilotcurrent rises above or falls below predetermined limits for extendedperiods of time.

9. In combination, a signal transmission line, an amplifier on saidline, means for controlling the gain of said amplifier according to theenergy level of a pilot current on the line, a relay of limitedsensitivity having a contact arm held in operative position andrestoring means, and means operated by said relay in case of largevariations in energy level of the pilot current for preventing operationof said amplifier control means, for operating said relay restoringmeans to return the contact arm to initial position and for operating analarm when the large variations of pilot current level extend forprolonged periods of time.-

10. In combination, a signal transmission line, an amplifier on saidline, means for controlling the gain of said amplifier according to theenergy level of a pilot current on the line, a relay of limitedsensitivity having a contact arm held in operative position whenoperated and restoring means, means operated by said relay in case oflarge variations in energy level of the pilot current for preventingoperation of said amplifier control means, means controlled by andoperated a predetermined interval after operation of the relay foroperating said restoring means to return the contact arm to initialposition, and means controlled by said relay for operating an alarm incase of large change in level by the pilot current for a prolongedperiod.

11. In combination, a signal transmission line, an amplifier on saidline, means for controlling the gain of said amplifier according to theenergy level of a pilot current on the line, a relay of limitedsensitivity having a contact arm held in operative position whenoperated and restoring means, a motor operated by said relay in case oflarge Variations in level of the pilot current, means operated by saidrelay for preventing operation of said amplifier control means, meansoperated by said motor after a predetermined delay for operating saidrelay restoring means, and means operated by said motor in case of aprolonged large change in level of the pilot current for operating analarm.

12. In combination, a signal transmission line, an amplifier on saidline, means for controlling the gain of said amplifier according to theenergy level of a pilot current on the line, a relay of limitedsensitivity having a contact arm held in operative position whenoperated and restoring means, a motor, means operated by said relay incase of large variations in the energy level of said pilot current forpreventing operation of said amplifier control means and for operatingsaid motor, means comprising a cam operated by said motor for insuringrotation of the motor for a predetermined period, means comprising asecond cam operated by said motor for operating said restoring means andfor preventing operation of said amplifier control means, and meanscomprising a third cam operated by said motor for operating an alarm incase of large changes in energy level of the pilot current for anextended period.

13. In combination, a signal transmission line, an amplifier on saidline, means for controlling the gain of said amplifier according to theenergy level of a pilot current on the line, a relay of limitedsensitivity having a contact arm held in operative position whenoperated and restoring means, a motor, means comprising a control relayoperated by said relay of limited sensitivity in case of large energyvariations of said pilot current for operating said motor and forpreventing operation of said amplifier control means, means comprising acam member operated by said motor for holding the motor circuit closedfor a predetermined period, means comprising a second cam memberoperated by said motor to become eifective after a time interval foroperating the restoring means of said relay of limited sensitivity andfor again operating said control relay, and means comprising a third cammember operated by said motor to become effective after said second cammember for operating an alarm in case of a large variation in energylevel of the pilot current for a prolonged period of time.

14. In combination, a signal transmission line, an amplifier on saidline, means for controlling the gain of said amplifier according to theenergy level of a pilot current on the line, a relay of limitedsensitivity having a contact arm held in operative position whenoperated and restoring means, a control relay for operating saidrestoring means, a thermal element in the energizing circuit of saidcontrol relay for effecting operation of said control relay when heatedfor a predetermined period, means operated by said relay of limitedsensitivity in case of a large change in voltage of said pilot currentfor preventing operation of said amplifier control means and for heatingsaid thermal element to operate said control relay, and means comprisinga second thermal element heated under control of said control relay incase of large voltage change of the pilot current for a prolonged periodfor operating an alarm.

DONALD M. TERRY.

